1. Field
The present disclosure relates to methods of extracting distance information and optical apparatuses using the methods, and more particularly, to methods of extracting distance information about a distance between an object and an optical apparatus by using an actual nonlinear waveform, as opposed to an ideal waveform, and optical apparatuses using the methods.
2. Description of the Related Art
Research on three-dimensional (3D) cameras or laser radars (LADARs) for obtaining distance information about a distance between an object and an optical apparatus has recently increased. In general, distance information about a distance between an object and an optical apparatus may be obtained by using a stereo vision method that uses two cameras or a triangulation method that uses structured light and a camera. However, the stereo vision method and the triangulation method have disadvantages, in that the accuracy of distance information drastically decreases as a distance between an object and an optical apparatus increases. Further, it is difficult to obtain precise distance information because both methods are dependent on the state of a surface of the object.
In order to overcome the disadvantages of the stereo vision method and the triangulation method, a time-of-flight (TOF) method has been suggested. The TOF method involves emitting a laser beam to an object and measuring a time taken for light reflected from the object to be received by a receiving unit. In more detail, the TOF method involves projecting light having a specific wavelength, for example, light having a near infrared (NIR) wavelength of 850 nm, to an object by using a light-emitting diode (LED) or a laser diode (LD), receiving part of the light having the wavelength of 850 nm reflected from the object by using a receiving unit, and processing the received light by using the receiving unit to generate distance information. The TOF method may be performed in various methods according to a process of processing light.
For example, the TOF method using a shuttered light pulse (SLP) modulates an image reflected from an object by using an image intensifier or a solid-state modulation device, captures the modulated image by using an image sensor, and obtains distance information from an intensity value of the captured modulated image. The TOF method using the SLP requires a very high light modulation rate of tens to hundreds of MHz to determine a phase shift of light according to distance and a light travel time. To this end, for example, the TOF method using the SLP uses an image intensifier that includes a multi-channel plate (MCP) or a GaAs-based solid-state modulation device. In recent years, an improved GaAs-based modulation device and a thin film modulation device using an electro-optic material have been developed.
Examples of the process of processing light for distance information extraction include a method of pulse-driving a light source and a light modulation device, a method using a special waveform, such as a triangular or ramp waveform, and a method using a sinusoidal waveform. Also, various methods of driving a light source and a light modulation device according to used waveforms have been developed, and various algorithms for extracting distance information from obtained intensity values have been developed.
The TOF method extracts distance information on the assumption that used waveforms, for example, a triangular waveform and a sinusoidal waveform, are ideal waveforms. However, actually, it may be difficult to have ideal waveforms due to nonlinearity and operation errors of the light source or the light modulation device. For example, an LED operates at a current greater than a threshold current, and output optical power has saturation and nonlinearity even during operation of the LED. Also, a gain of the light modulation device is nonlinear in many cases during operation of the light modulation device. In particular, if a high voltage or a high current is used, more nonlinearity or operation errors may occur according to a design of a driving circuit. Such operation errors directly affect distance information extraction, thereby causing an error. Accordingly, an expensive waveform generator that generates a very accurate ideal waveform, or a complex additional algorithm and a driving circuit for correcting the error, may be required.